The technology for constructing current-age crushed stone-based roadways and pavements has progressed, for example, from the multi-size stone-based roadways of Macadam in the early 19th Century to the highly mechanized paving industry of today using stone-based mixtures in combination with binders such as cementious or organic material. Paving technology with concrete mixtures historically has evolved with improved cement formulations and working tools. These tools have functioned to consolidate concrete mixes, for example by vibration to remove air pockets or voids while retaining requisite air entrainment. Additionally, tools have been evolved to cause concrete mixes of design varying plasticity to conform to the boundary-defining constraints of a mold. Further, tools and associated structures have been developed to form a specified surface texture and appearance of a given pavement. Process efficiencies for producing concrete paving commenced to be substantially enhanced in the mid-1950's with the initial development of the slipform paver.
The slipform paver is a driven track mounted platform functioning as a dynamic mold which moves over site-or roadbed deposited concrete in the manner of a moving extruder to form continuous pavement. To carry out this molding process, a process scheme involving a linear sequence of stages is involved which, in part, emulates a working combination of earlier concrete working tools. In this regard, an initial stage of the slipform machinery continuously spreads the newly mixed plastic concrete mass before molding stages, whereupon energy is applied to this incompressible distributed mass in the course of treatment to mold a pavement.
Energy application to the concrete mass involves the dynamic contributions of vibration, squeeze or pressure, and mold surface movement. All of these energy related contributions are associated or interact with the mix attributes of the concrete itself, which not only are the product mix formulation, but also may be subject to the logistic vagarires of concrete supply and delivery. Any changes in these variables without accommodation in the process may well affect paving results either positively or negatively.
The consistency of a concrete mix generally is a function of its specified formulation or design. Mix plasticity typically is measured by the classical slump evaluation. For the process of slipform machine molding of concrete, consistency as developed from formulations and/or molding dynamics is highly important in the achievement of paving success. As a preliminary aspect, the slump of a mix must be low enough to produce sharp and stable pavement edges, yet the mix must respond well to the extrusion process and consolidate properly. Currently, concrete mixes intended for slipform paving will exhibit a one to two inch slump to meet these requirements.
The concrete forming dynamics of the slipform molding procedure generally commence following the spreading of concrete which has been deposited on a prepared roadbed. Spreading conventionally is accomplished, inter alia, with a rotating, horizontally disposed auger, often performing just forwardly in the process line from a horizontally disposed metering or strike-off component. A volume of concrete thus is disposed extending across the roadbed within a region sometimes referred to as a "grout box". It is at this position in the continuous extrusion process that the concrete mass is confronted by the extrusion envelope, the crucial point where machine performance essentially determines the quality of the final product. At this location, the concrete mass is treated by internal vibration which is induced into the mix by an array of hydraulically driven vibrators, the vibrational components of each of which typically are mounted within a case of cylindrical cross-section. Aligned in parallel with the path of pavement formation, the vibrators preferably are mounted at an elevation over the roadbed representing about the mid point or center of the thickness of the ultimately created pavement. Generally implemented as a rotational, hydraulically driven eccentric, the vibrator generates frequencies in a range of about 9,000 to 10,500 rpm or vpm (vibrations per minute).
These internal (within the concrete mass) vibrators serve two critical purposes in the pavement molding process: to consolidate the concrete mass by removing undesirable voids; and to fluidize the material to an extent facilitating paver slip mold movement over and through the concrete mass. In effect, an individual intra-aggregate particle friction reduction permits the "slipping" required for the slipform process. Such fluidization also functions to enhance a uniformity of pressure of the mix at the confronting nose or entrance of the slip mold. In this regard, intra-aggregate particle friction is reduced by this fluidization at the process location where the concrete is sheared off at its proper molded elevation by the nose or front of the profile pan or mold. Changes in pressure at this confronting point will cause the slipform machine to compensate by lifting or diving. Since the profile pan cannot compensate adequately for large changes in pressure at the nose, a non-uniform surface and non-uniform consolidation may be the deleterious result. Further, the angle at which the nose of the profile pan shears concrete, also known as the angle of attack, is of importance in this molding process. Inasmuch as mix characteristics will determine how much energy the machine needs to inject into the concrete to successfully complete the extrusion process, the best angle of attack will be different for different concrete mixes. Some slipform paver configurations also will incorporate a tamper bar just in front of the nose of the pan or mold. This tamper bar functions to carry out a secondary consolidation on the concrete mix and aids in the performance of the profile pan by moving large aggregates to just below the pavement surface at this critical shear point. The thin layer of mortar created by the tamper bar at the surface between the concrete and the profile pan lowers friction between the mold and the concrete to aid in the overall molding process.
From the foregoing, it may be observed that the performance of the arrayed vibrators within the slipform process is of substantial importance. Investigators of this aspect of the dynamic molding process have determined that each of the vibrators of the submerged or invested array creates a conically-shaped zone of influence. The extent of this zone changes as the level of energy, correlated with frequency or vibrations per minute (vpm) changes. An increase in the frequency of vibration tends to widen the zone, while a corresponding decrease narrows it. The vibration energy level required for a particular mix design and depth of placement may require changing the number of vibrators in the array and operating them at higher or lower energy levels. In mounting the vibrators in the noted array across the nose or opening of the mold, spacings of each vibrator are set on what is speculatively anticipated as a slight overlap of each conical zone of influence. Typical vibrator spacings on slipform paving machines range from 12 to 24 inches. Those vibrators which are located next to the edges of the mold typically are set approximately six inches from each side form of the machine and to achieve an optimum orientation of the noted cone of influence. The cylindrical housings of the vibrators, the surfaces of which are in contact with the concrete, are set in a somewhat horizontal orientation. As is apparent, the height of concrete over the vibrators evokes a hydrostatic head. Any changes in the head will develop concrete pressure changes to affect the attributes of vibration and slipform paver machine performance. Thus, the array of vibrators provides an important aspect of the energy application to the concrete mix within the slipform process and, as is apparent, an ongoing evaluation of the performance of the vibration function is an important operational parameter in achieving a successfully molded pavement product.
Traditionally, the quality of vibrator performance and attendant concrete paving quality has been predicated upon the experience and skill of the slipform machine operator. These operators, relying on their experience, control such dynamic parameters as machine speed and the rotational speed of the consolidation vibrators. Generally, the operator has performed under the assumption that all vibrators in an array are operating at any given time, at the same rotational rate. Rotational speed deviation or vibration stoppage has been discerned generally with the ear and eye of the operator who stands in the midst of the rigorous ambient environment of the slipform paver. These vibrator rotational rates must be established to effectively remove entrapped air voids while maintaining an air entrainment of about 6% of the mass volume of the concrete. The latter air entrainment is a mix design feature for accommodating ambient weather-temperature variations as the pavement is utilized over its lifespan. Where the applied vibrational energy is inadequate, a key aspect of the slip molding process is lost with product degradation. On the other hand, where applied vibrational energy is excessive, a disassociation of the important mix of different aggregate size occurs, the larger stone being driven from the vibration region. This mix discontinuity contravenes industry teaching reaching at least as far back as Macadam. More specifically, as the vibrator is moved through a concrete mix, it creates a hole in its orbit. Should the slump of the concrete be too greatly reduced by energy induced therein, the adjacent material will not fill the hole left behind the orbiting vibrator and consolidation is compromised. The phenomenon now is known as "post-holing". Typically, the resultant void then is filled with smaller aggregates and the mix discontinuity has been observed in the finished pavement as longitudinal visible texture changes or markings referred to in the industry as "vibrator trails". Recent investigation of this phenomenon has revealed that the trails exhibit the noted mix discontinuity with an absence of larger aggregate components and a tendency to early pavement deterioration.
Individual vibrator breakdowns generally have been discerned by observing the external affect on the mix or by hearing a vibrator bearing deteriorate. Rotational speed variations of the eccentrics of the vibrators are difficult to observe or hear and, thus for some industry participants, tachometers have been employed to monitor vibrator speeds to aid the operator in discerning performance vagaries.
Over the recent past, additional vibration systems have been incorporated within slipform paving machinery. Traditionally, periodically positioned expansion joints have been created in concrete pavement by placing preassembled reinforcing rod "baskets" across the roadbed at joint locations ahead of the paving process. The pavers then encountered and paved over these assemblies, whereupon expansion joints were formed over them. Important production cost reduction has been realized through the utilization of a dowel bar inserter which is mounted upon the paving machine. In general, a plurality of reinforcing bar dowels are loaded within a distribution component mounted upon the machine, which dowels are engaged by a placement mechanism and inserted down within the formed paving in parallel, horizontally and at given lateral spacing, for example upon 18 inch centers. In general, the procedure is designed such that the dowels are positioned in a manner providing for no relative movement between the dowels and the pavement during their placement. Metal forks mounted in a hydraulically powered insertion frame are used for such positioning. As the insertion sequence progresses, the dowels are released to the surface of the pavement, held in horizontal position by spacer plates and hydraulically implanted in the concrete. Vibration is added as the forks engage the dowels and penetrate the concrete. The dowels then are released at the proper elevation in the concrete structure and the forks then retract. As the forks retract, vibration is induced to close the hole created by the penetration and reconsolidate concrete surrounding the dowel. It is important that the dowels be located in parallel alignment with the longitudinal extent of the pavement and remain in a horizontal orientation, a feature not always accomplished. Further, the reconsolidation procedure must not be deleterious to the mix structure. As is apparent, a control over the vibratory aspects of this dowel insertion technique is of paramount importance. Of course, the optimization of the insertion of vibration energy is important for any aspect of concrete placement including the use of hand-held mix vibrating devices.
As may be apparent, although many advances have been made in the slipform paving machines since their introduction in the 1950's, much improvement in their performance and operation is called for. Such improvements necessarily must look to the somewhat delicate balance of concrete mix design, delivery, vibration dynamics, linear machine speed, and machine geometry adjustments.